1. Field of the Invention
The present invention generally relates to the interconnection of electronic components, particularly micro-electromechanical systems (MEMS), and more specifically to methods of fabricating packages for MEMS devices to provide external electrical connections, e.g., to printed circuit board leads.
2. Description of Related Art
As modern electronic devices have become more complicated, it has also become more difficult to interconnect their various components. For example, the physical size of microprocessors and integrated circuits (ICs) continues to shrink, but the number of leads provided on such chips is increasing due to added functionalities. The smaller size of the chips creates a problem when trying to provide connections between the chip leads and external devices or leads, such as those on a printed circuit board (PCB).
A state-of-the-art IC chip might be as small as one or two square centimeters, but have as many as 100 or more circuit leads. Typical interconnect spacing for the external leads is now on the order of 100-150 microns, and is expected to become even finer (e.g., 50 micron pitch). To reduce costs of assembly, semiautomatic or automatic insertion of components onto PCBs is often employed. Minor errors in the placement of these chips can accordingly result in mis-connection, leading to nonfunctional units, and oftentimes damage to sensitive electrical circuits within the chips. These problems can be compounded on multi-chip modules (MCMs).
A variety of interconnection packages have been devised to assist in the placement of electronic devices. The package for a semiconductor device typically fulfills three functions. First, it provides environmental and physical protection for the silicon chip. Second, it provides a means to facilitate handling of the chip. Third, it provides electrical connections from the chip to the system in which it is installed. Packages are usually soldered to their circuit boards to physically and electrically connect the package to the circuit board. Other types of interconnection can be provided, such as optical or fluid ports.
An additional level of packaging in the form of a socket is sometimes used to electrically and physically connect the IC package to its printed circuit board. Sockets for IC packages are usually fitted with pins which are soldered to a circuit board. Pressure contacts can also be used, in which case the socket is pushed against the circuit board with screws, springs, or some other type of mounting hardware. In the latter configuration pressure contacts must have enough compliance or conformance to compensate for non-planarities present in the circuit board and/or the mounting surface of the IC package.
One exemplary IC package is disclosed in U.S. Pat. No. 5,413,489. In that design, an integrated circuit die is mounted onto the upper surface of a multi-layer ceramic carrier, or spreader. A conventional solder-bump flip-chip (xe2x80x9cC4xe2x80x9d) process is used to connect the die to the substrate spreader. Interconnections can also be achieved using wire bonding, tape automated bonding (TAB), or elastomeric interconnects. The spreader is a multi-layered ceramic carrier, with vias forming connections between the layers. The bottom surface of the spreader has an array of contacts. A shell or cap is affixed to the spreader, surrounding and protecting the die. The spreader is placed in a molded plastic socket cover. The spreader and cover are further mounted on a socket base. The base has posts adapted to fit into corresponding holes of the circuit board.
Similar packaging and electrical interconnection considerations apply to micro-electromechanical systems (MEMS). In the field of miniaturization, it is not only electronic devices that have shrunk, but mechanical structures as well. MEMS devices are very small systems that are fabricated with technologies much like those used to fabricate integrated circuits, but MEMS devices interact with their environment in more ways than a traditional IC. MEMS devices typically have physical structures or mechanisms on an upper surface that perform the desired interaction with the environment, e.g., mechanical, optical or magnetic interactions.
MEMS devices may include very small electromechanical components such as switches, mirrors, capacitors, accelerometers, inductors, capacitive sensors and actuators that combine many of the most desirable aspects of conventional mechanical and solid-state devices. Unlike conventional mechanical devices, MEMS devices can be monolithically integrated with integrated circuitry, while providing much improved insertion loss and electrical isolation over solid-state devices. Typically, the MEMS devices are anchored to and suspended above the substrate so that they can move. For example switches open and close, variable capacitors are trimmed or tuned, actuators move back-and-forth and accelerometers deflect. Oftentimes these devices perform multiple functions or are simultaneously subjected to more than one signal. For example, low frequency signals are used to open and close MEMS switches and trim or tune variable capacitors while they conduct a high frequency AC signal. Mechanical actuators respond to an electrostatic force produced by a low frequency signal while functioning as an actuator. Accelerometers deflect in response to acceleration forces and in turn can modulate an AC signal. One example of a MEMS device is the micromachined fluid sensor disclosed in U.S. Pat. No. 5,969,259. In that design, side-ports are added to a dual in-line (DIP) type IC package, to provide fluid communication with sensors located inside the device.
The structures in MEMS devices are often quite robust when considered within the framework of their small size, but are very fragile relative to the macro-world of conventional IC packaging systems. Additional problems can arise relative to these devices, such as electrostatic and surface-tension induced attraction. Microscopic contamination can add to these problems and cause the device to fail when they induce detrimental electrostatic or surface-tension related attraction.
Surface micromachining, modified surface micromachining and frontside silicon-on-insulator (SOI) techniques are among those used to fabricate suspended MEMS devices. Surface micromachining uses standard deposition and patterning techniques to build-up the MEMS device on a substrate. Deposition and patterning techniques can also be used to build up the device on the surface of a substrate. MEMS devices are often fabricated such that the functional mechanism of the device remains buried within a sacrificial oxide material that is still present at the end of the xe2x80x9cfront-endxe2x80x9d processing. At some suitable time prior to use or being completely packaged, the MEMS devices are released. The process of etching or otherwise removing the material that encases the components is often referred to as xe2x80x9creleasingxe2x80x9d. It is a common practice to saw a semiconductor wafer into individual dies prior to their xe2x80x9creleasexe2x80x9d, so that particles from the sawing operation are less prone to foul the MEMS mechanisms. It is also common for the MEMS foundry to ship the die in the unreleased state. Shipping the MEMS die while still encased in its sacrificial oxide material (and having the foundry customer do the release of the device) helps reduce contamination of the device during shipping and handling.
While the use of a release layer is very desirable to protect the delicate MEMS components, this approach creates further problems during xe2x80x9cback-endxe2x80x9d processing by the final manufacturer/assembler. The MEMS device cannot be fully (i.e., hermetically) sealed prior to release, and so can still become contaminated or damaged during installation. It would, therefore, be desirable to devise an improved method for handling the MEMS die and performing the release operation. It would be further advantageous if the method could utilize packaging which provided a cost-effective and space-efficient means of connecting the MEMS die to external electrical/electronic signals, while maintaining a hermetic cavity for the MEMS mechanism.
It is therefore one object of the present invention to provide an improved method of interconnecting a micro-electromechanical system (MEMS) device to external circuitry.
It is another object of the present invention to provide such a method which facilitates the back-end release process of the MEMS die.
It is yet another object of the present invention to provide a hermetic package for MEMS devices.
The foregoing objects are achieved in a method of fabricating a flex circuit interconnect subassembly for use with a MEMS device, generally comprising the steps of placing a flex circuit on a pad insert, wherein the flex circuit has lead portions adapted to interconnect with bond pads formed on an electronic device die, attaching a carrier insert to the pad insert to deflect the lead portions of the flex circuit from a first plane toward a second plane, and applying a cover insert to the pad insert, after the attachment of the carrier insert, to re-deflect the lead portions of the flex circuit toward the first plane. The flex circuit interconnect subassembly may be combined with an electronic device die/carrier subassembly to form a completed electronic device package. The invention is particularly suited for making such an electronic device die/carrier subassembly which has a micro-electromechanical system (MEMS) die permanently affixed to a carrier. The carrier is advantageously utilized during the process of releasing a protective coating from the surface of the MEMS die which support the various MEMS components. In a preferred implementation the method uses a pin plate fixture, wherein the flex circuit is applied to the pin plate fixture, and then forced against the pad insert using the pin plate fixture. The carrier insert is attached to the pad insert while the flex circuit is applied to the pin plate fixture. The flex circuit interconnect subassembly is combined with the electronic device die/carrier subassembly after removing the pin plate fixture from the flex circuit and pad insert. The MEMS device package may have a cover providing various features for the MEMS devices, and may further have an overmolded body to provide a hermetic seal.